1. Technical Field
The present invention relates to a treating agent for liquid crystal alignment to be used for liquid crystal devices such as a liquid crystal display device, and a liquid crystal alignment film and a liquid crystal device utilizing it. More particularly, it relates to a treating agent for liquid crystal alignment, which is excellent in the printing properties to a substrate, which will be free from separation from the substrate during rubbing, which provides an alignment film which will be less likely to be damaged by rubbing, and which provides an excellent voltage holding property when a liquid crystal cell is operated, and a liquid crystal alignment film and a liquid crystal device employing it.
2. Background Art
A liquid crystal display device is a display device utilizing an electrooptical change of liquid crystal, and as an apparatus, it is small in size and light in weight and has an attractive feature such that the power consumption is small. Accordingly, in recent years, it has undergone remarkable developments as a display device for various types of displays. Among them, an electric field effect type liquid crystal display device of twisted nematic type (TN type) is a typical one, wherein nematic liquid crystal having positive dielectric anisotropy is employed, liquid crystal molecules are aligned at the respective interfaces of a pair of electrode substrates disposed to face each other, in parallel with the substrates, and the substrates are combined so that the alignment directions of liquid crystal molecules will cross each other. Further, among the TN type liquid crystal devices, an active matrix operation system utilizing a TFT (Thin Film Transistor) having excellent display performance, has been actively developed.
In such a TN type liquid crystal display device, it is important that long axes directions of liquid crystal molecules are uniformly aligned in parallel on the substrate surface and further that liquid crystal molecules are aligned with a certain tilt angle to the substrate. As a method for aligning liquid crystal molecules in such a manner, a rubbing treatment has been usually employed. The rubbing treatment is a method wherein an organic coating film is formed on the surface of a substrate, and the surface is rubbed with a cloth of e.g. cotton, nylon or polyester in a predetermined direction, so that liquid crystal molecules are aligned in the direction of rubbing. By this method, stabilized alignment can be obtained relatively easily, and industrially, this method is mainly employed. As the organic film, polyvinyl alcohol, polyoxyethylene, polyamide or polyimide may, for example, be mentioned. However, from the viewpoint of chemical stability, thermal stability, etc., polyimide is most commonly employed.
With respect to the treating agent for liquid crystal alignment using a polyimide, commonly, a varnish having a solvent-soluble type polyimide dissolved in a solvent is coated on a substrate followed by baking to form a polyimide film, and then rubbing treatment is carried out thereon to obtain a liquid crystal alignment film, or a solution of a polyamic acid as a polyimide precursor, is coated on a substrate, generally followed by baking at a temperature of at least 150.degree. C. for imidization, and then rubbing treatment is carried out thereon to obtain a liquid crystal alignment film.
As basic properties required for such an alignment film, the tilt angle of liquid crystal molecules, the voltage holding ratio and charge-accumulation property by direct current voltage, may, for example, be mentioned, and the voltage holding property is an important property from the viewpoint of the constitution of the device, particularly in the TFT display system. It has been known that such properties of an alignment film greatly depend on the liquid crystal species to be used, but they are influenced also by the structure of the polyimide to be used. Accordingly, it has been attempted to improve film properties by variously selecting the structure of the polyimide.
With respect to the liquid crystal alignment film formed by a solvent-soluble polyimide, it has been known that usually baking can be carried out at a low temperature and excellent voltage holding property can be obtained. However, there has been such problems that the tilt angle is not adequate or its stability is not adequate, or the charge-accumulation property is not adequately low. Further, there has been such a problem that e.g. the printing properties and adhesive properties to a substrate are poor.
On the other hand, the liquid crystal alignment film using a polyamic acid as a polyimide precursor, has such advantages that a high and more stable tilt angle can be obtained and the charge-accumulation property by the direct-current voltage can be made small, and further, the printing properties and adhesive properties to a substrate are excellent. However, it has such drawbacks that the voltage holding property is poor, or the solvent resistance at the surface of the film is poor in the case where the imidization degree is insufficient.
From the viewpoint of the production of a liquid crystal display device, properties such as adhesive properties and printing properties of the alignment film to a substrate, and rubbing resistance, are important. Particularly in rubbing treatment, which is an industrially employed method for treating liquid crystal alignment, there are such problems that the liquid crystal alignment film may be separated off from the substrate due to abrasion during rubbing, or the liquid crystal alignment film may be damaged, thus influencing over display properties.
Further, from the viewpoint of reliability and environmental resistance of the display device, an alignment treating agent having excellent alignment film properties, particularly voltage holding property, at a high temperature, has been required. Namely, a fluorine type liquid crystal is widely used for a display device of TFT operation system, and it has been known that usually the voltage holding property tends to be high in the case of using this liquid crystal. However, even in a case where the voltage holding ratio is high at the beginning, a display failure such as display unevenness may be caused during a long-term use of the liquid crystal display device in some cases, and the reliability is not adequately high. Further, in the case where liquid crystal for low-voltage operation is used, the voltage holding property may decrease, and an adequately high reliability can hardly be obtained, such being problematic.
As mentioned above, both solvent-soluble polyimide and polyamic acid have merits and demerits which are in an antinomic relation to each other as a liquid crystal alignment film, and it is not necessarily easy to satisfy all properties required as a liquid crystal alignment film. However, a treating agent for liquid crystal alignment, which is particularly excellent in the printing properties to a substrate, the adhesive properties and rubbing resistance, and which has a high reliability, has been desired.
The present invention has been made to overcome the above problems. Namely, it is an object of the present invention to provide a treating agent for liquid crystal alignment, which is excellent in the adhesive properties and the printing properties to a substrate, which is free from separation from the substrate during rubbing, which provides an alignment film which is less likely to be damaged by rubbing, and which provides an excellent voltage holding property regardless of the type of liquid crystal when a liquid crystal cell is operated.